Remove the restriction on multi-user PRs

This came up in #docker-dev today.  We're not sure exactly why the original restriction was put it place.  It is not well obeyed.  Please comment if you know the origin of the restriction.

Docker-DCO-1.1-Signed-off-by: Timothy Hobbs <timothyhobbs@seznam.cz> (github: timthelion)
Merge pull request #7354 from timthelion/patch-8

Remove the restriction on multi-user PRs